The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with magnetic resonance imaging and will be described with particular reference thereto.
Heretofore, magnetic resonance imagers have commonly included a series of annular resistive or superconducting magnets. Vacuum dewars in superconducting magnets and housing structures of resistive magnets have defined a central, longitudinal bore within which the subject was received. Commonly, a series of gradient magnetic field coils were mounted to a cylindrical dielectric former which was mounted in the magnet bore, reducing the patient receiving diameter. A whole body RF coil was mounted on another dielectric former which was mounted in the interior bore of the gradient coil dielectric former, further reducing the patient receiving diameter.
The diameter reductions become more critical when self-shielded gradient coils are used. With self-shielded gradient coils, there are two sets of gradient coils disposed in a spaced relationship. The pair of gradient coil sets produce magnetic fields which (1) sum within the bore to create the desired magnetic field gradients and (2) subtract outside the bore. The subtraction zeroes the field to inhibit magnetic field gradient pulses from inducing eddy currents in the main magnet and associated structures. To achieve this shielding effect efficiently, significant minimum spacing between the primary and secondary gradient coils is required. Analogously, an RF shield is advantageously disposed between the RF coil and the gradient coils to prevent the RF pulses from inducing eddy currents in the gradient coils. Again, a significant, minimum spacing between the RF coils and the RF shield is required.
A large patient aperture is advantageous. A large patient receiving bore not only accommodates large patients and provides a less claustrophobic environment, but it also allows imaging of portions of the subject further from the center of the bore. For example, shoulder imaging requires the patient's shoulders to be displaced radially inward from the RF coil. On the other hand, a large patient aperture has associated costs such as a reduced sensitivity of the RF body coil, a lower gradient performance, and more costly magnets.
Mounting the RF body coil to a continuous dielectric cylindrical former reduces the patient receiving diameter to that of the former. In a birdcage-type RF coil, there are typically a plurality of longitudinally extending foil strips mounted to the dielectric former. Even although the foil strips are displaced by significant distances around the circumference, the dielectric former is solid and continuous between them. Not only does this continuous cylindrical dielectric former reduce the bore, it also obstructs the use of specialty coils such as insert head or biplanar gradients. These insert coils need to be secured to the inner diameter of the gradient tube for structural stability. The intervening RF coil cylinder obstructs direct access to the gradient coil dielectric former and the foil strips of the RF coil limit where interconnections can be made.
The present invention provides a new and improved RF coil assembly which overcomes the above-referenced problems and others.